Flame synthesis of hydrogen cyanide



May 13, 1952 c, MoKlNNls 2,596,421

FLAME SYNTHESIS OF HYDROGEN CYANIDE Filed April 12, 1948 Patented May13, 1952 UNITED STATES PTENT OFFICE FLAME SYNTHESIS OF HYDRQGEN CYANIDEArt 0. Mciiinnis, Long Beach, Calif.,,assignor to Union Oil Company ofCalifornia, Los Angeles, Calif., a corporation of CaliforniaApplicationApril 12, 1948, Serial No. 20,489

1 "Y @laims.

This invention relates to a process for the synthesis of hydrogencyanide from a gaseous mixture containing ammonia, oxygen and ahydrocarbon. More particularly it pertains to the synthesis of hydrogencyanide by flame reactions in which contact times such as 1x10 secondsor less are conducive to high conversions. Hydrogen cyanide is anindustrial commodity having large and widely varied uses. This materialis used in metallurgical operations involving metal smelting andplating, as a military poison, as a reagent in the synthesis of manysynthetic dyestuffs, as a reagent in the chemical industries forpreparing acrylonitrile, pharmaceuticals,

various chemical intermediates and cyanhydrins,

in the preparation of oxy-acids from aldehydes and ketones, in foodindustries as a fumigant in treating grain elevators and as adisinfectant for flour and other foods, as an insecticide in warehousesfor combating rats and vermin in ships, as a general fumigant andinsecticide for citrus and other fruit trees, in the rubber industry. asan accelerator in the coagulation of latex, in textile industries forthe treatment of raw cotton, and in agriculture where it is used as asoil disinfectant and 'as a general parasiticide.

Hydrogen cyanide has been prepared by a wide variety of chemicalreactions some of which have become commercially practical. Onecommercial production method involves the reaction of hydrogen andammonia with either carbon monoxide, carbon dioxide or acetylenecatalyzed by the presence of heated platinum. A second commercialoperation is the reaction between nitric oxide and a hydrocarbon underthe influence of a hot platinum-rhodium catalyst. The third commerciallysuccessful method is that involving the reaction of ammonia, oxygen anda hydrocarbon in the presence of a platinum catalyst in which a hydrogento ammonia ratio of between 3 to 1 and 5 to 1 is employed.

The threeprocesses named above are the principal ones employed in theUnited States for hydrogen cyanide production and the principaldisadvantage to these is the requirement of an expensive and an easilypoisoned platinum metal catalyst. In general, these processes requireextensive installations of complex equipment.

The process of the present invention overcomes most of the seriousdifficulties inherent in the conventional processes in that no catalystis required, thus eliminating poisoning problems, the conversions are ashigh as or higher than those previously obtained, and the raw materialsare readily available. The elimination of expensive catalysts and thesimplicity of the equipment are outstanding advantages. The process ofthe present invention also generally involves the liberation of heat inwhich little if any extraneous heat is needed to maintain the reaction.The reaction may be carried out if desired Without preheating thereacting gases.

It is an object of this invention to provide an improved process for thenon-catalytic preparation of hydrogen cyanide by a flame reaction.

It is another object of this invention to provide an imprcvedprocess forthe production of hydrogen cyanide by flame synthesis which involves areaction zone adapted to the maintenance of thin flames in whichchemical reactions may be effected at contact times of less than about ll0- seconds and at high reaction rates.

Other objects and advantages of the present invention will becomeapparent to those skilled in the art as the description thereofproceeds.

Briefly, the present invention comprises in its preferred modificationthe preparation of a gaseous mixture containing ammonia, oxygen and ahydrocarbon which is preferably of low molecular weight and normallygaseous. This gaseous mixture may be preheated by indirect exchange withthe hot efliuent gases or introduced into the flame synthesis reactor atatmospheric temperature. The reaction of the reactant gases in the flamereactor goes approximately according to the following equation in whichmethane is assumed to be the hydrocarbon:

This reaction does not proceed appreciably according to the aboveequation without the maintenance of a flame of thepropercharacteristics.

Attempts to effect this reaction of ammonia, oxygen, and methane in aheated tube Without the maintenance of a flame have resulted innegligible conversions to hydrogencyanide. It is believed that thesuccess of the process according to this invention is attributable toefiecting the conversion of ammonia andmethane to hy= drcgen cyanide atelevated temperatures and at an extremely-short contact time of theorder of 1x10 seconds through the medium of flame synthesis in whichthin flames, are maintained. The yields of hydrogen cyanide, thusobtainedare at least equal to or exceed those realized in theconventional processes employing expensiverare metal catalysts.

A gas, also herein termed a reactor effluent gas, is thus produced whichcontains hydrogen cyanide and water vapor together with nitrogen and asmall amount of unreacted ammonia. In the preferred modification, thisefiiuent gas is cooled and introduced as feed gas to a selectiveadsorption column wherein the hydrogen cyanide is recovered from thegaseous mixture as a substantially pure fraction. Unreacted ammonia isalso recovered and recirculated to the reactor with fresh ammonia feed.Ammonia conversions of greater than 90% to hydrogen cyanide have beenthus obtained.

In flame reactions, the flame temperature causes very high chemicalreaction rates so that in many cases the primary desired reaction oc--curs almost completely during the short contact time. Undesiredby-products form through degradation of the desired product or of thereactant materials. The rate of flame propagation, which is determinedin turn by the combustion rate or the ignition rate of the combustiblemixture, determines the maximum allowable space velocity in thesynthesis reactor. This consideration together with the physical shapeand size of the flame directly determines the contact time obtained fora given reaction. Cylindrical or cone shaped flames give long contacttimes and thin flat flames permit very short times, the latter beinghighly desirable in the process of this invention.

The present invention is primarily directed to the short contact time,high temperature flame synthesis of hydrogen cyanide from a gaseousmixture containing ammonia, oxygen, and a low molecular weighthydrocarbon.

The process may be more fully described and understood by reference tothe accompanying drawings in which;

Figure 1 represents a schematic flow diagram of the preferredmodification of the combined flame synthesis and selective adsorptionseparation process for the production of hydrogen cyanide,

Figure 2, Figure 3 and Figure 4 represent cross sections of suitableapparatus in which the flame reaction may be carried out.

The following description of Figure 1 may be considered as a practicalexample of one modification of this invention for the synthesis of hydrogen cyanide by a non-catalytic flame reaction.

Referring now more particularly to Figure 1, ammonia is introduced at arate of 11.9 mols per hour through line it controlled by valve H at arate determined by flow controller I? and is introduced by means of linei3 into reactant gas mixing manifold it. The low molecular weighthydrocarbon in this modification is methane, although other lowmolecular weight hydrocarbons such as ethane, propane, butane, andhigher, as well as the unsaturated low molecular weight normally gaseoushydrocarbons such as ethylene, acetylene, propylene, and the like, maybe used. The hydrocarbon is introduced by means of line l5 controlled byvalve It at a rate of 11.9 mols per hour determined by flow controllerI? and is passed through line 58 into manifold Id. The source of oxygenin this particular example is atmospheric air which is introducedthrough line [9 controlled by valve 20 at a rate of 76.2 mols of air perhour determined by flow controller 2!. This quantity of air contains16.0 mols of oxygen. The air is then passed by means of line 22 intoreactant gas manifold M. Reactant gas manifold I l may comprise a packedtube, and may be provided with a series of ofl I and manifold 23 intothe reactor.

The degree to which the reactant gases are preheated, if at all, islargely dependent upon the composition of the reactant gases andparticularly upon the oxygen concentration, which in turn depends uponthe source of the oxygencontaining gas, and may vary from atmosphericperature to as high as about 800 C. The oxygen-containing gas thusemployed may be air, oxygen enriched air, or substantially pure oxygen.Any of these oxygen sources may be employed under proper reactionconditions for the efficient conversion of ammonia to hydrogen cyanidein the presence of methane. It has been found that the oxygen to ammoniaratio in particular affects the ammonia conversion and determineswhether or not a flame may be maintained. As the oxygen to ammonia ratiois decreased, i. e. the oxygen concentration of the reactant gases isreduced, a point is reached at which the flame is extinguished. Atincreased reactant gas preheat temperatures the critical oxygen toammonia ratio at flame extinction decreases, thus permitting loweroxygen to ammonia ratios to be employed. The theoretical oxygen toammonia ratio for this reaction is 1.5 and the ammonia to methane ratiois 1.0. When introducing the reactant gases at room tempera- 1 tureusing air as the source of oxygen, the flame may not be maintained at anair to ammonia ratio of less than 2.2 which is well above thetheoretical. Upon preheating the reactant gases to a temperature ofabout 800 C. in a system having the same methane to ammonia ratio of1.0, the critical air to ammonia ratio at flame extinction decreases toabout 1.2 which is below the stoichiometric ratio in the theoreticalequation. Similar relations exist for the higher molecular weighthydrocarbons, but the ratios are difierent. When the source of oxygen isair or other gas of approximately the same oxygen concentration, thereactant gas mixture is advantageously preheated to temperatures betweenabout 300 C. and 800 C. With oxygen enriched air containing betweenabout 20% to about oxygen as the oxygen source, preheat temperatures offrom room temperature to as high as about 600 C. may be used. When pure,or relatively pure, oxygen is used, no preheating is required tomaintain the flame althoughthe gases may be preheated to between roomtemperature and about 300 C. if desired.

The reactant gases pass from inlet manifold into reactor 31 wherein acontrolled flame reaction is effected. The hot eiiluent gases areremoved via line 32 and pass through interchanger 26 wherein heat may beindirectly transferred to the entering gases. These gases are thenpassed through line 33 into cooler 3 where they may be further cooled tothe dew point or below depending upon the nature of the hydrogen cyanideand ammonia recovery methods applied.

This cool reactor eflluent may be passed by means ofline35 and line 36controlled by valve 3 line 38 and. line 39, controlled by valve 40 intohydrogen cyanide recovery vessel 4| in which hydrogen cyanide may beremoved in a variety of ways as hereinafter more fully described. Inanother modification, the cooled reactor effluent gas containing nocondensate may be passed by means of line 35, line 42 and line 43controlled by valve 44 and line 45 into selective adsorption column 46wherein unreacted ammonia, ifpresent, and the hydrogencyanide productare recovered as substantially pure and separatefrac tionsas hereinaftermore fully described.

The reactor eiiiuent may be cooled, if desired, to below the dew pointto effect a partial separa tion of thewater presentsincethere are 3*molsof water vapor for every mol of hydrogen cyanide formed in the reactor.In this event the cooled efiluent passes by means of line-35, line 42,and line 41 controlled by valve 43 into separator. 43. The condensate,which consistsof a diluteaqueous solution of hydrogen cyanide containingsome dissolved ammonium cyanide, collects in the bottom of separator 49and is removed therefrom through line 50 controlled by valve 5| which inturn is actuated by liquid level controller 52. The cooled butuncondensed portion of the efiluent is removed from the upper portion ofseparator 49 by means of line 53 and may pass throughlines 3S and 39controlled by valve 4|) into hydrogen cyanide recovery vessel 4|, or itmay pass through lines53 and 3B and thence through line 54 controlled byvalve 55 and through line 45 intcselective adsorption column 46.

Referring now more particularly to hydrogen cyanide recovery vessel 4|several methods of recovery are applicable. i In one modification,vessel 4| may comprise a bubble plate or other similar type ofabsorption column and the eifiuent gases may rise upwardly therethroughcountercurrent to a downwardly flowing liquid absorbent. The liquidabsorbent is introduced by means of line 56 controlled by valve 51 intothe upper portion of vessel 4|.

In one modification, this absorbent comprises anaqueous solution of abasically reacting compound such as an alkali metal or alkaline earthmetal hydroxide or the like. An aqueous solution containing dissolvedalkali or alkaline earth metal cyanide salts is thus formed in vessel 4|and is removed from the bottom thereof by means of line 5'! controlledby valve 58 which in turn is actuated by liquid level controller 59. Theundissolved gases substantially free of hydrogen cyanide are removedfrom the upper portion of vessel 4| by means of line 60 controlled byvalve 6|. This overhead gas contains unreacted ammonia which may beseparated if desired by continuous selective adsorption and returned tothe flame reactor.

In a second modification, recovery vessel 4| may contain a solid packingof an alkali metal or alkaline earth metal'hydroxide in which case thepassage of the hydrogen cyanide-containing gases results in theconversion of at least a part of the hydroxide to the corresponding.metal cyanide. If desired, the metal hydroxide may be employed in aseries of vessels as static beds, one being on stream while the othersare being unloaded or loaded. The overhead gases produced'in thismodification also contain unreacted ammonia which may be recovered andrecirculated with fresh ammonia feed to the flame rea t rma .WlI WQiflQatiPP! reqqrerw s syldt ay employ water as the absorbent in whichcase both the ammonia and the hydrogen cyanidearere; covered togetherproducing an aqueous solution containing hydrogen cyanide and ammoniumcyanide.

In a fourth modification, vessel 4| may comprise a bubble tray tower inwhich anacid extractant is employed to remove ammonia from the reactoreffluent allowing the hydrogen cyanide to pass overhead for removalthrough line 6!! controlled by valve 6|. Dilute inorganic acids may beemployed such as hydrochloricor sulfuric or carbonic acid. Solutions ofacidsaltsmay, be used advantageously, particularly the ammoniumacid'salts such as ammonium acid sulfate and the ammonium acidphosphates. Inthis modification the overhead gas maybe separated bycontinuous selective adsorption or by other means indicated above fortheseparation of the hydrogen cyanide in substantially pure form.

Inthe preferred modification, however, the reactor effluent is cooled inproduct cooler 34 to a temperature above the dew point of the gas andthe mixture is passed by means of line 35 and line 42 controlled byvalve 44 through lines43 and 45 into selective adsorption column 46 forseparation as described below.

The continuous selective adsorption process is based upon preferentialadsorption phenomena exhibited by solid adsorbents in which certaingaseous constituents are more strongly adsorbed than are others. In thisparticular gaseous mix ture, comprising thereactor effluent, hydrogencyanide and water vapor are the most readily adsorbable, ammonia is ofintermediate adsorbability, and methane, hydrogen, nitrogen and carbonmonoxide which, when present, are the least readily adsorbableconstituents. This permits a highly eiilcient separation of the desiredproduct from the gaseous mixture and also a separationof the unreactedammonia in substantially pure form for recirculation. Unusually highconversion efficiencies are thus obtained.

Selective adsorption column 45 is provided, at successively lower levelstherein, with adsorbent hopper 62, cooling zone 63, lean gas disengagingzone 54, adsorption zone 5.5, feed gas engaging zone 66, primaryrectification zone 61, side. out gas product disen aging zone 68,secondary rectification zone 6%, rich gas disengaging zone l'll,steaming zone ll, heating zone 12, adsorbent feeder zone 13 and bottomzone 74. The adsorbent passes downwardly by gravity successively throughthe aforementioned zones as a moving bed and is removed from bottom zone74 through transferline l5 and is introduced into lift line i6. Herein agaseous suspension of adsorbent is formed with a lift gas passedthereinto by lift gas blower l1 and transfer-line Hi and the lift gassuspension is passedupwardly into impactless separator 19. Herein thelift gas suspension is broken and the adsorbent and lift gas passdownwardly-through transfer line 84 as substantiallyindependent phasesinto hopper d2 wherein the'adsorbent collects. The lift gas then passesout of the upper-portion of column 46 by means of line 8| and isreturnedby means of line 82 to lift gas blower H for recirculation.

The gaseous mixture to be separated passes by means of line 45 into feedgas engaging zone '56,. and then upwardly through adsorption zone 65wherein the perferential adsorption of the most readily adsorbableconstituents occurs. he e d east v dsorbent. r tio v ries .Wi v

type of adsorbent and the concentration of adsorbable constituents inthe mixture. With charcoal as the adsorbent and a typical effiuent gas,between 150 and 350 pounds of charcoal per thousand standard cubic feetis generally adequate. In this case'a rich adsorbent containing adsorbedhydrogen cyanide, water vapor and ammonia is formed leaving asubstantially unadsorbed lean gas containing hydrogen, nitrogen, carbonmonoxide and methane substantially unadsorbed. A portion of this gas isremoved from lean gas disengaging zone 64 as a lean gas prodnot by meansof line 83 and is passed into separator 84 in which suspended particlesof adsorbout are separated. The lean gas product is removed therefrom bymeans of line 85 controlled by valve 85. The remaining portion of thislean gas product passes upwardly through the tubes of cooling zone 63wherein it serves to saturate the downwardly flowing adsorbent with theconstituents of the lean gas and to dehydrate the adsorbent. Thisportion of the lean gas joins the lift gas stream and is removedtherewith by means of line 8!. To avoid accumulation of the lift gasrecycle a portion of the lift gas is continuously removed from line 8!through line 3? controlled by valve 38. This gas comprises a mixture ofhydrogen, nitrogen, and carbon monoxide which has a substantially lowermethane content than does the lean gas product. This lean gas may beemployed as a source of pure hydrogen and nitrogen for ammoniapreparation or may be discarded or burned. V

The richadsorbent formed in adsorption zone E5 passes downwardly intoprimary rectification zone 6'! wherein it is contacted by a first refluxgasrrich in ammonia. Since this constituent is more readily adsorbablethan methane and the less readily adsorbable constituents, apreferential adsorption of ammonia occurs forming a partially rectifiedadsorbent and causing the desorption of small quantities of constituentsdesired in the lean gas product. These desorbed constituents pass upwardand are removed with the lean gas.

The partially rectified adsorbent thus formed passes downwardly intosecondary rectification zone 69 wherein the partially rectifiedadsorbent is contacted with a second reflux gas rich in hydrogen cyanideand water vapor and again preferential adsorption causes the desorptionof ammonia forming a rectified charcoal. The temperatures maintained insecondary rectification zone are above the complete dissociationtemperature of ammonium cyanide; thus a substantially completeseparation is permitted. The wet ammonia thus desorbed collects in sideout gas disengaging zone 38 from which a portion passes into primaryrectification zone 61 as said first refiux gas. The remaining portion isremoved by means of line 89 and is introduced into separator 95 whichserves to separate suspended particles of adsorbent. The ammonia sideout gas, substantially free of less readily adsorbable constituents andof hydrogen cyanide and water, is removed from separator 26 through line9! controlled byvalve 92 and is sent to storage or further processingfacilities, not shown, by means of line 83 controlled by valve 34%, orin the preferred modification is returned by means of line 95 controlledby valve 9% to the ammonia inlet line Ill, wherein it is combined withfresh ammonia and introduced subsequently into flame reactor 3i.Ammonium cyanide dissociates completely at temperatures below theboiling point of water and the adsorbed constituents, i. e., ammonia,hydrogen cyanide, and water, are readily separable by heating the richcharcoal to temperatures above the complete dissociation temperature.

The rectified charcoal formed in secondary rectification zone 69 passesdownwardly into steaming zone H wherein the adsorbent is contacted withstripping steam effecting the preferential desorption of adsorbedhydrogen cyanide. A substantially complete hydrogen cyanide desorptionis effected therein and the partially stripped adsorbent passesdownwardly through the tubes of heating zone 12 wherein the adsorbent isindirectly heated by means of circulating flue gas or condensing vaporssuch as steam or mixtures of diphenyl and diphenyl oxide to atemperature of about 500 F. Additionalquantities of steam are introducedby means of line 91 controlled by valve Q8 into bottom zone M to passupwardly through the tubes of heating zone 12 to efiect a substantiallycomplete stripping of the remaining quantities of. hydrogen cyanide. Thehydrogen cyanide together with water vapor collects in rich gasdisengaging zone 10 from which a portion passes into secondaryrectification zone 69 as said second reflux gas, while the remainingportion is removed from zone "it by means of line 99 and is passed intorich gas cooler Hill. The rich gas may contain as high as 75% or higherof hydrogen cyanide, the remaining material being water vapor. In onemodification, cooler Hm merely cools this rich gas product to a liquidat its bubble point and this liquid is transferred by means of line it!controlled by valve In into hydrogen cyanide distillation column 133wherein the rich gas product is distilled to separate an overheadproduct of hydro" gen cyanide and leaving a bottoms product comprisingwater. Other well known methods may be employed to prepare anhydrousacid. A portion of the bottoms product is passed through reboiler IM tosupply heat to the bottom of the column while the remainder is removedby means of line Hi5 controlled by valve I06. The overhead productpasses through line it! through condenser 38 and a portionofthe'condensate thus formed passes through. line H 19 as reflux to columnm3. The remainder of the overhead product passes to production by meansof line Hi3 controlled by valve Ill and consists of substantially purehydrogen cyanide. method for separating the components of the productgas by preferential adsorption is de scribed and claimed in myco-pending application, Serial No. 200,263, filed December 11, 1950.

Referring now to Figures 2, 3 and 4, cross sectional drawings of threemodifications of flame reactors are shown in which flame reactions maybe effected.

The reactor shown in Figure 2- comprises a cylindrical vessel I29provided with reactant gas inlet I21 and efiluent outlet 122. Thebehavior of this simple burner depends to some extent upon thedimensions. It was noted that in reactors of decreasing diameter,maintenance of the flame became increasingly difilcult. The flame, uponwhich the reaction depends, tended to draw away from the burner wallsforming an annular void between the flame and the wall of vessel I20. Inreactors of larger diameter, the transverse area occupied by the flamecompared to the transverse area of the vessel is greater. This isreflected in an increase in the quantity of ammonia converted tohydrogen cyanide-since A preferred,

a" greater proportion of reactant gas passes through the flame.

Packing I23 is provided through which the reactant gases pass atvelocities greater than the flame propagation rate prior to enteringflame I24 and a thorough mixing is achieved. This packing may comprise abed of glass helices, sand, glass beads, porcelain pellets, or a poroussintered glass plate. With each type of packing, the flame tends to drawaway from all adjacent solid surfaces. The mixed reactant gases passthrough packing I23 into reaction zone lilac wherein reaction flame I24is maintained at a gas velocity less than the flame propagation rate.The product gases are quenched in reaction zone I24a.

Referring now more particularly to Figure 3, a reactor is shown whichcomprises vessel I25 provided with reactant gas inlet I20, jacket gasinlet I21, and efliuent gas outlet I20. Vessel lZt may be cylindricaland reactant gas inlet I20 may be cylindrical and concentric with vesselI25 at one end providing annular space IBM. The outside diameter D1 ofreactant gas inlet I20 is at least 80% and preferably between 90% and99.5% of inside diameter D2 of vessel I25, although smaller relativediameters may be employed. A packing may be employed in reactant gasinlet if desired. Flame I29 is maintained in reaction zone I 30a whereinthe product gases are quenched and rapidly cooled from the flame temperature through absorption of the heat of for mation of the product.Flame I30 is annular in shape and is maintained by a combustible tureintroduced into annular space I200; via jacket gas inlet I21. Thepresence of, annular flame I30 eliminates the drag effect of the vesselwalls on synthesis flame I20. The heat loss from flame I29 through thevessel walls is also eliminated thus permitting flame I29 to assume adiameter equal to that of reactant gas inlet I25. By so operating, asubstantially complete elimination of reactant gas by-passing aroundsynthesis flame I29 results causing materially increased conversionsover those obtainable with other types of burners.

In the synthesis of hydrogen cyanide from a hydrocarbon, ammonia, andoxygen, the reactant gas mixture containing methane, ammonia, and airfor example is passed through reactant gas inlet I26 to maintainsynthesis flame I20. A gaseous mixture of hydrocarbon, which may benatural gas for example, and air is introduced into annular space I29aand jacket flame I30 is thereby maintained. The temperature of flame I20is quite high, between 1000 C. and 1500 C., but the gas products arerapidly quenched through absorption of heat in the formation of hydrogencyanide, and the efliuent is removed at a temperature of from 200 C. to500 C.

With the apparatus shown in Figure 3 it has been possible to achievehydrogen cyanide production efficiencies of greater than based upon thequantity of ammonia reactet. This apparatus also permits a substantiallyreduced degree of reactant gas preheat.

Another modification of apparatus which may be employed as a flamereactor is shown in cross section in Figure 4. This apparatus comprisesvessel I3I provided with reactant gas inlet I02 and effluent gas outletI33. Vessel I3I is further provided with heating means I 34 which maycornprise, as shown in this figure, an electrical element I35 withconnectors I30 and i 31, or it may comprise a Jacket by means ofwhichheat is introduced from a circulating fluid. Through the application oflocalized heating, the flame is maintained in area I38 which, aspreviously shown, otherwise tends to draw away from the walls of vesselI3I. In cylindrical vessels the flame in area I38 is circular and verythin having the shape of a disc.

The maximum hydrogen cyanide conversions are found when oxygen toammonia ratios of from 0.8 to about 1.8 and ammonia to hydrocarbonratios of from 0.75 to about 6.0 are employed in preparing the reactantgas mixture wherein the hydrocarbon contains up to about 6 carbon atomsper molecule. These ratios are valid also for higher molecular weighthydrocarbons as well. Since methane in the form of natural gas isreadily available in large volume, this hydrocarbon is somewhatpreferred over the others. For methane, or natural gas containing orbetter methane, oxygen to ammonia ratios of from 1.2 to 1.8 may be usedand preferably a ratio of about 1.5. The ammonia to hydrocarbon ratiosin this instance may be from 0.75 to 1.25 althougha ratio of about 1.0is preferred.

When employing methane as the source of hydrocarbon and air which may beenriched with oxygen as the source of oxygen, it is desirable to form areactant gas mixture containing between about 5% and 20% methane, from5% to 20% ammonia, and from about 7.5% to 30% oxygen, the remainderbeing nitrogen. This gaseous mixture is preferably preheated to atemperature of between about 300 C. and 800 C. or higher. When employingpure oxygen in the reactant gas mixture, a reactant gas containingbetween 20% and about 40% by volume methane, 20% to 40% by volumeammonia, and 30% to 60% by volume of oxygen may be used. It ispreferable however to use 25% to 35% of ammonia and methane and 30% to50% oxygen. With hydrocarbons of higher molecular weight than methane,the concentration of hydrocarbon is lowered to compensate for increasedquantities of carbon per mol of gas. In the foregoing description thehydrocarbon mentioned has been methane. However, any normally gaseoushydrocarbon including both the saturated and unsaturated normallygaseous hydrocarbons may be employed or mixtures of these hydrocarbonsmay be used as indicated in the accompanying examples. For example.natural gas is well suited to the process of this invention. Saturatedhydrocarbons such as methane, ethane, propane and the butanes may beemployed as well as unsaturated hydrocarbons, such as acetylene,ethylene, propylene, and the like, and in general the normally aseoushydrocarbons havin up to about 6 carb n atoms er molecule may be used.The normally licuid hvdrocarbons such as nentane. hexane. etc. benzene,cyclohexane and the like may be employed as the source of active carbonin the flame synthe is ,reactions by first vaporizing these in a suitale vaporizer and mixing the resultant vapors in the correct proportionswith ammonia and oxygen to form a suitable reactant gaseous mixture.

The fact that such a wide range of hydrocarbcns may e em oyed in thepractice of this invention gives this process another outstandingadvantage over the conventional catalytic methods for hydrogen cyanideproduction. The rare metal catalysts, such as platinum, are very easilypoisoned by hydrocarbons of higher molecular weight than methane and byother gaseous constituents such as carbon monoxide, hydrogen sul- OxygenSource Component Pure Atmospheric Oxygen Air Hydrocarbon -.percent -30l-l5 Ammonia 25-45 -17 Oxygen; do. 35-50 10-20 As a typical example ofthe process accordin to this invention the following data are given:

Example I A gaseous mixture similar to that employed in Example I waspassed into the reactant gas inlet tube oi an apparatus similar .to thatshown in Figure 3 and the mixture of methane and air was passed into theannular space surrounding this inlet tube so that the hydrogen cyanideproduction flame was surrounded by a cylinder of burning methane andair. Hydrogen cyanide yields of from 65% to '75 based on the quantity ofam-. monia reacted were obtained.

Example III A gaseous mixture of methane, ammonia and oxygen analyzing29% ammonia, 29% methane, and 42% oxygen by volume at room temperaturewas passed through an apparatus similar to that shown in Figure 3 inwhich the hydrogen cyanide flame was surrounded by an annular flame ofburning methane and oxygen. A yield of 75% hydrogen cyanide based on thequantity of ammonia reacted was obtained.

' Example IV V 7 A gaseous mixture containing 10.9% methane,

10.9% ammonia, 16.4% oxygen, and 61.8% nitro-' gen was preheated to atemperature of 800 C. and passed into a flame synthesis reactor similarto that shown in Figure 3. Methane and air were also introduced to forma cylinder of flame about the disc shaped central flame formed by thereactant mixture. A 91.0% conversion of ammonia to hydrogen cyanideresulted.

Example V A gaseous mixture analyzing 28% ammonia, 27% natural gas, and45% oxygen was introduced into the reactant gas inlet and natural gasand air was introduced into the jacket flame inlet of a reactor similarto that shown in Figure 3. A steady easily controlled flame wasmaintained without preheating the reactant gases. An

. 12' ammonia-hydrogen cyanide conversion'of 89.0% was obtained.

Example VI A reactant gas mixture containing 3.0% ncrmal hexane vapor, V16.0% ammonia, 17.0% oxygen, and the remainder nitrogen was prepared andintroduced into the reactor shown in Figure 3 at a preheat temperatureof 550 C". An ammonia conversion to hydrogen cyanide of 73% was effectedbased on the quantityoii ammonia consumed.

Erample VII A flame reactor eiiiuent obtained from the con-' version ofa gaseous mixture of natural gas, am-: monia, and air had approximatelythe following composition:

' Per cent Nitrogen 55.0 Hydrogen cyanide 9.0 Ammonia 4.0 Methane 3.0Carbon monoxide 3.0 Carbon dioxide 2.0 Water vapor 24.0

This material was introduced into 'a continuous selective adsorptioncolumn wherein it was contacted with between 150 and 250, pounds or"activated vegetable charcoal per 1,000 standard cubic feet of theeiiiuent gas. A rich gas product was produced from the selectiveadsorption column at a rate of about 350 mols per 1,806 mols of efiluentgas feed. This product contained about 25 mol per cent of hydrogencyanide and was cooled to form a liquid at its bubble point. Thisliquidwas introduced into a distillation column from which an overheadproduct comprising 99% hydrogen cyanide was produced. A rectified sideout gas product was produced simultaneously from the selectiveadsorption column at a rate of about 38 mols per 1,000 mols of effluentgas feed. This side cutgas product analyzed higher than mol per centammonia and was returned with fresh ammonia feed to the flame generator.A small amount of stripping steam was introduced into the desorptionzone of the selective adsorption column to insure substantially completedesorption of the adsorbed hydrogen cyanide and water. A temperature of500 F. was employed as a maximum tempera turc'in the desorption zone. Inthis combination process a 92% conversion of ammonia to hydrogen cyanidewas obtained.

Particular embodiments of the present invention have been hereinabovedescribed in considerable detail by way of illustration.- It should beunderstood that various other modifications and adaptations thereof maybe made by those skilled in this particular art without departing fromthe spirit and scope of this invention as set forth in the appendedclaims.

I claim: a

1. A process for the production of hydrogen cyanide which comprisesforming a reactant gas mixture essentially comprising between about 10and about 1'7 per. cent by volume of ammonia, between about 1 and about15 per cent by volume of a hydrocarbon containing up to 6 carbon atomsand between about 10 and about 20 per cent by volume of oxygen in theform of air; passing said mixture into a synthesis flame maintained bythe combustion of said mixture; burning a combustible gas mixturecomprising a hydrocarbon and oxygen to form an auxiliary flamesurrounding .said synthesis flame, whereby said synthesis flame iscaused to take the form of a thin flat disc, the dimension of which inthe direction of its propagation is such that the time of passage of thereactant gas mixture through said synthesis flame is less than about 110 second; and cooling the product gas to a temperature below that atwhich substantial reaction occurs immediately after its formation insaid synthesis flame.

2. A process for the production of hydrogen cyanide which comprisesforming a reactant gas mixture essentially comprising about one volumeof ammonia, between about 0.17 and about 1.33 volumes of a hydrocarbonselected from the class consisting of methane and natural gas, andbetween about 0.8 and about 1.8 volumes of oxygen; passing said mixtureinto a synthesis flame maintained by the combustion of said mixture;burning a combustible gas mixture comprisin a is methane.

hydrocarbon and oxygen to form an auxiliary 5 flame surrounding saidsynthesis flame, whereby said synthesis flame is caused to take the formof a thin flat disc, the dimension of which in its direction ofpropagation is such that the time of passage of the reactant gas throughsaid synthesis flame is less than about 1 1O-Vsec- 0nd; and cooling theproduct gas to a temperature below that at which substantial reactionoccurs immediately after its formation in said synthesis flame.

3. The process of claim 2 wherein the hydro- 4. The process of claim 2wherein the hydrocarbon component of the reactant gas mixture is naturalgas.

5. The process of claim 2 wherein the oxygen is in the form of air.

6. The process of claim 2 wherein the auxiliary flame is maintained bythe combustion of a combustible gas mixture comprising air and ahydrocarbon selected from the class consisting of methane and naturalgas.

7. The process of claim 2 wherein the reactant gas mixture is preheatedto a temperature between about 300" C. and about 800 C. prior to itspassage into the synthesis flame.

ART C. MCKINNIS.

REFERENCES CITED The following references are of record in the file ofthis patent? UNITED STATES PATENTS

